Bicuspid aortic valve (BAV) is associated with premature valve dysfunction and abnormalities of the ascending aorta. Limited data exist regarding serial changes of aortic dilation in patients with BAV. We studied paired transthoracic echocardiograms of 68 patients with BAV (mean age 44 years) and with at least 2 examinations >12 months apart (mean follow-up 47 months) to characterize the progression of aortic dilation and the natural history of valve function. We measured aortic root and ascending aortic diameters at baseline and follow-up. We measured aortic gradients and severity of aortic regurgitation (AR). During follow-up, aortic diameters increased at the sinuses of Valsalva by 1.9 mm (95% confidence interval [CI] 1.3 to 2.5), at the sinotubular junction by 1.6 mm (95% CI 0.8 to 2.3), and at the proximal ascending aorta by 2.7 mm (95% CI 1.9 to 3.6). Mean rate of diameter progression was 0.5 mm/year at the sinuses of Valsalva (95% CI 0.3 to 0.7), 0.5 mm/year at the sinotubular junction (95% CI 0.3 to 0.7), and 0.9 mm/year at the proximal ascending aorta (95% CI 0.6 to 1.2). Progression was observed regardless of hemodynamic function at baseline. Mean aortic valve gradient increased significantly from baseline to follow-up (17.6 mm Hg vs 25.7 mm Hg, p <0.001). The degree of AR increased during follow-up in 17 patients (25%). In addition, progression of aortic diameter dilation occurred irrespective of baseline valve function in adult patients with BAV. We also observed considerable progression of aortic gradients and AR over time.